Due to a continuing need for the development of rugged and reliable sensors capable of taking measurements in harsh industrial environments, there has been extensive research in this area as evidenced by the technical literature.
For example, Yamazoe et al. has reported a series of sensors based on stabilized zirconia with mixed metal oxide electrode systems. A CdCr2O4 attached device was reported as a good potentiometric sensor for NOX gases in the temperature range of 500 to 600° C. In several other papers, they have studied different metal oxide systems and found CdMn2O4 as a good candidate for sensing applications. They have also proposed a NOX sensing mechanism involving mixed potential based on the measurements of polarization curves.
Lu et al. has recently reported the study of YSZ (yttria stabilized zirconia) based sensors using a tungsten tri-oxide (WO3) electrode for the detection of NO and NO2. The EMF response of the WO3-attached device is nearly linear to the logarithm of NO or NO2 concentrations. Kurosawa et al. has fabricated a NOX sensor based on MgO stabilized zirconia with an auxiliary phase of Ba(NO3)2. E. L. Brosha et al. has reported mixed potential sensors based on dense, thin films of lanthanum cobaltate pervoskites for carbon monoxide (CO) and hydrocarbon gases. R. Mukundan et al. has studied mixed potential YSZ and CeGdOX based sensors with platinum (Pt) and gold (Au) electrodes for hydrocarbon and CO sensing measurements. According to them, a CeGdOX based sensor gave a more stable and reproducible response than a YSZ electrolyte due to the better oxygen reduction kinetics of metal electrodes on ceria-based electrolytes. The same group has reported that the reproducibility of the response behavior was dependent of Au morphology. T. Hibino et al. has reported non-Nernstian behavior at tantalum oxide modified Au electrodes for hydrocarbon sensing.
Zeolites have recently become the subject of considerable research in sensor applications. In the past 15 years there has been considerable research done on zeolite modified electrodes (ZMEs), the study of putting a layer containing zeolite particles onto an electrode surface. Walcarius classifies five main applications of ZMEs, which include electrocatalysis, electroanalysis, charge storage devices, molecular recognition, and mass transport characterization. For utilization in sensor materials, electroanalysis is of particular importance. Walcarius sub-divides this application into the five categories of direct detection, indirect detection, amperometric biosensors, potentiometric analysis, and voltametry after preconcentration. Various methods have been used to cover the electrode such as zeolite dispersion in a binder, pressing zeolite powder onto the electrode, applying a coating of the zeolite in a polymer matrix, and covalently linking zeolite to the surface of the electrode. The majority of electroanalysis applications using ZMEs have been for determination of species, usually metal cations, in the liquid phase.
There have been few accounts of using zeolite materials for gas phase sensing at various temperatures and conditions. One design studied is a sensor operating at 300° C. for the detection of CO using SnO2 coated onto a platinum wire. The SnO2 is impregnated with Au—La2O3 and this layer is subsequently covered with a layer of the zeolite ferrierite. The addition of the zeolite serves as a catalyst filter to allow selectivity for CO in the presence of H2, CH4, C2H4, i-C4H10 and C2H5OH.
Au—NaY zeolite electrodes are used to monitor ethanol and ammonia vapors using cyclic voltametry. High current densities are obtained in the presence of ethanol and ammonia vapors at 25° C.
Zeolites deposited on a quartz crystal microbalance (QCM) are used as sensors for gaseous molecules. Cu-ZSM-5 zeolite is deposited onto a quartz substrate with a gold (Au) electrode and used to detect NO in helium (He) at 384K. The shift of the fundamental resonance frequency of the QCM was found to be proportional to the amount of NO present. A similar study done at 423K involving a thin layer of the zeolite faujasite on a QCM with Au electrodes detected SO2 in the presence of O2. Other studies involving sensor based systems include: the use of zeolites in amperometric biosensors for H2O2 and glucose oxidase, NH3 detection by monitoring the change of conductivity of zeolite Na+ beta and H+ beta measured by impedance spectroscopy, use of the zeolite NaY—(Ru2+(bpy)3) as a fluorescence O2 sensor, and for gas detection by micromechanical cantilevers attached with zeolite crystals at the apex.
There is a continuing need for the development of rugged and reliable sensors capable of making measurements in the harsh industrial environments found in the steel, heat treating, metal casting, glass, ceramic, pulp and paper, automotive, aerospace, and utility and power industries. The 1990 Clean Air Act amendments (CAAA) will require many power and utility industries to monitor emissions. Emission monitoring sensors for these applications include those for CO, NOX, O2 and hydrocarbons. Combustion engines are a major contributor of NOX emissions. The major species of NOX in automotive exhaust gases are NO, NO2 and N2O of which 90% of the total amount is NO. Nitrogen oxides can be toxic to humans, with possible lung impairment due to exposure of less than 15 ppm NO2. It is therefore imperative to develop a sensor for NOX that will provide real time analysis for engine control and onboard diagnostics to monitor and control these emissions.